In an assembled battery where a plurality of energy storage devices are connected in series, due to the difference in initial capacity, difference in degree of degradation, or the like, between the energy storage devices, there may be a case where variation (difference in charge amount) is generated between charge amounts of the energy storage devices. When variation occurs in charge amount between the energy storage devices, at the time of charging, there is a possibility that the energy storage device which has a large charge amount becomes an overvoltage. Conventionally, there has been known a technique which suppresses a state where an energy storage device becomes an overvoltage by setting a charge voltage lower than a predetermined voltage (see Japanese Patent No. 4461114).
However, when a charge voltage is set low, a charge amount which can be charged or charge efficiency is lowered. It is also conceivable that a state where an energy storage device becomes an overvoltage can be suppressed by performing an equalization control for reducing a difference in charge amount between the energy storage devices or controlling a charge voltage such that an energy storage device does not become an overvoltage. In this case, however, a device for performing an equalization control or a voltage control is required. In addition, there is an energy storage device having a characteristic that a voltage is suddenly increased near a full charge state. In the energy storage device having such a characteristic, there is also a possibility that an equalization control or a voltage control cannot be performed in time.